EP0573353B1 - High temperature composite material having honeycomb structure and process for its preparation - Google Patents

Abstract

A honeycomb structure made of a high-temperature structural composite material comprising a fibrous reinforcement texture is produced from a three-dimensional fibrous texture formed by stacked two-dimensional layers (40) connected together by means of fibres passing through the layers. Staggered cuts in the form of slits (42) are formed through the layers, throughout the entire thickness of the texture, and the cut texture is stretched (drawn) in the direction not parallel to the cuts (42) but parallel to the layers (40) in order to form cavities (46) whose walls are constituted by the lips of the cuts (42). The cut texture, held in the stretched state, is densified by the material which makes up the matrix. <IMAGE>

Description

The present invention relates to the manufacture of nest structures of bees in thermostructural composite material.

Thermostructural composite materials are characterized by their mechanical properties which make them suitable for constituting structural elements and by their ability to maintain their mechanical properties up to high temperatures. Typical thermostructural composite materials are carbon-carbon composite materials (C-C) and composite materials with ceramic matrix (CMC).

C-C composites are made up of a reinforcing texture, or preform, carbon fiber densified by a carbon matrix. CMCs are consisting of a preform of refractory fibers (carbon or ceramic fibers) densified by a ceramic matrix. A commonly used ceramic material used for the manufacture of CMC is silicon carbide (SiC).

The preform of a C-C composite or a CMC is produced by stacking or layering of unidirectional strata (layers of parallel wires or cables between them) or multidirectional (layers of fabric, veils of fibers, layers felt), or by winding threads, ribbons or strips, or by three-dimensional weaving. In the case of layering of strata, these can be linked between them by needling, by sewing, or by implantation of transverse threads. The preforms are made from carbon or ceramic fibers or, more generally, from fibers into a carbon or ceramic precursor, the transformation of the precursor being carried out after the necessary textile operations in the production of the preform.

The densification of the preform aims to fill the accessible porosity of this by the material of the matrix. This densification can be carried out by impregnation of the preform by means of a liquid phase containing a precursor of the matrix material, followed by a transformation of the precursor, or by chemical vapor infiltration.

The techniques mentioned above for developing preforms carbon or ceramic fibers and densification with a carbon matrix or ceramics are well known.

There are several methods of making nest structures bees.

A first known method (FIGS. 1A, 1B and 1C and JP-A-1 119 574) consists in stacking and glue staggered sheets 10. Gluing is done along strips 12 parallel, the glue strips located on one side of a sheet being offset by compared to those located on the other side (Figure 1A). The set of leaves is cut into sections 14, perpendicular to the adhesive strips. Each section is then stretched in a direction normal to the faces of the leaves (arrows f of the Figure 1B) to give, by deformation, hexagonal cells 16 (Figure 1C). A honeycomb panel 18 is thus obtained, on either side of which can be glued a metallic or composite sheet.

Such a method is used to make honeycomb structures metallic. The sheets 10 are cut from a metal strip and the cells 16 are produced by plastic deformation of the metal.

This process can also be implemented using sheets of cardboard or paper. In the latter case, after staggered stacking and gluing, the sheets of paper can be impregnated with a resin, for example a resin phenolic. The crosslinking of the resin is carried out after formation of the cells (this before or after cutting the set of sheets into sections).

To make a honeycomb structure from composite material thermostructural, it could be envisaged to implement a process of same type with two-dimensional fibrous strata, for example strata stacked and staggered fabric; densification and thereby stiffening of the structure being carried out after stretching and formation of alveoli. Each layer should normally be made up of several layers of fabric, which implies the need for a bond between the layers to avoid their mutual separation during stretching. In addition, a bonding operation in staggered is difficult to achieve on fabrics with regularity and precision wanted so that at the time of stretching a tearing of the fabric due to a local defect does not happen. In addition, during the densification operation after stretching, it is to fear that thermal stresses would lead to destruction of the collage.

A solution consisting of sewing staggered layers of fabric, rather than sticking them, would avoid some disadvantages, but would pose significant difficulties in implementation.

A second known method (FIGS. 2A, 2B) consists in using fluted sheets, for example in metallic foil. Grooved sheets 20 are superimposed and glued or welded along their facets 22 in contact mutual (Figure 2A). Honeycomb panels 28 are directly obtained by cutting the block of sheets 20 perpendicular to the grooves (Figure 2B).

This process can make honeycomb structures in composite material using fluted sheets themselves of material composite. Such sheets can be obtained by draping layers of fabric by giving them the desired fluted shape and densification, for example by draping and molding of fabric layers impregnated with a resin. However, it it is then necessary to ensure an effective bonding of the fluted sheets capable of withstanding the service temperatures at which materials thermostructural can be brought to undergo. In addition, the operations of prefabrication of fluted sheets are long and expensive, which weighs down considerably the cost price of the honeycomb structure.

Finally, a third known method (FIGS. 3A and 3B) uses a sheet 30, for example a metal sheet in which cutouts are formed 32. The cuts are made in staggered rows along parallel lines (Figure 3A). The cuts are the same length and regularly spaced each other along each line. The cuts along a line are offset from those of the adjacent lines and each cut extends over a length greater than the distance between two neighboring cuts aligned. The sheet 30 is deployed by traction perpendicular to the lines of cutouts (arrows f 'in Figure 3A). By plastic deformation of the metal, it forms cells 36 at the cutout locations (Figure 3B). The deployment is limited so as not to induce, particularly at the ends of the cuts 32, stresses liable to cause tears in the leaf. The axis of each cell is inclined relative to the initial plane of the sheet at an angle of less than 90 °, so that the walls of the cells are not perpendicular to the general plane of the honeycomb panel 38 obtained.

This process is practically impossible to carry out on layers of fabric without causing tears in the fabric at the ends of the cuts at the time of deployment. In addition, this method has a limitation important as to the thickness of the honeycomb panel obtainable. Indeed, this thickness is determined by the distance between the cutting lines and this distance must remain small enough to allow training cells during deployment.

Also, the present invention aims to provide a method for to make a honeycomb structure in thermostructural composite material without encountering the above drawbacks.

In particular, the invention aims to provide a process thanks to which honeycomb structures of thermostructural composite material can be manufactured at a cost price moderate enough to open broad fields of applications.

• réaliser une texture fibreuse tridimensionnelle au moyen de strates bidimensionnelles superposées et liées entre elles au moyen de fibres traversant les strates,
• réaliser des découpes en quinconce en forme de fentes, à travers les strates, dans toute l'épaisseur de la texture,
• étirer la texture découpée en direction transversale par rapport aux découpes et parallèlement aux strates pour former des alvéoles dont les parois sont constituées par les lèvres des découpes, et
• densifier la texture découpée et maintenue à l'état étiré, par le matériau constitutif de la matrice, pour obtenir une structure en nid d'abeilles rigide en matériau thermostructural.

This object is achieved by a process for manufacturing a honeycomb structure in a composite material comprising a fibrous reinforcement texture densified by a matrix, the fibers of the reinforcement texture being in a material chosen from carbon and ceramics, as well as the matrix, process comprising the steps which consist in:

• achieve a three-dimensional fibrous texture by means of superimposed two-dimensional layers and linked together by means of fibers passing through the layers,
• make staggered slits in the form of slits, through the layers, throughout the thickness of the texture,
• stretch the cut texture in a transverse direction with respect to the cuts and parallel to the strata to form cells whose walls are formed by the lips of the cuts, and
• densify the cut texture and maintained in the stretched state, by the material of the matrix, to obtain a rigid honeycomb structure of thermostructural material.

The strata forming the reinforcing texture may be at least partially consisting of layers of fabric. The connection between the strata is carried out by example by implantation of threads, by sewing or by needling. In this last case, when the strata include layers of fabric, it may be advantageous to interpose between them strata formed of fiber veils, in order to provide fibers capable of being taken up by the needles and arranged through the strata during needling.

Fibrous reinforcement textures formed of two-dimensional layers superimposed and linked together for example by needling are well known.

The process according to the invention is particularly remarkable from the fact that a preform of a honeycomb structure is simply obtained by formation of staggered cuts and stretching of the texture.

This process differs from that illustrated in FIGS. 1A to 1C because that, according to the invention, the stretching is carried out parallel to the planes of the strata and not perpendicular to them.

The process according to the invention also differs from that illustrated by the Figures 3A and 3B. In fact, with this known process, the walls of the cells are formed by the parts of the sheet located between two lines of cuts. The deployment has the effect of causing a gradual inclination of these walls with respect to the initial plan of the sheet at the same time as the expansion of the leaf. This is not so with the process according to the invention. Stretching texture has the effect of spreading each other's lips apart from each form cells, the walls of which are formed by the lips of the cutouts. The thickness of the honeycomb structure is determined by the thickness of the fibrous texture and therefore does not experience the same limitation as in the case of process of FIGS. 3A and 3B where the thickness of the honeycomb structure is determined by the distance (necessarily limited) between two cutting lines neighbors.

The invention also relates to a honeycomb structure of composite material thermostructural as it can be obtained by the process defined more high.

According to the invention, such a structure, comprising a fibrous reinforcement texture densified by a matrix, is characterized in that the reinforcement texture is a three-dimensional texture formed by two-dimensional strata linked together by fibers crossing the strata, the alveoli of the honeycomb structure being formed across the strata.

Other particularities and advantages of the conforming process and structure to the invention will emerge on reading the description made below, as indicative and not limiting.

• les figures 1A, 1B et 1C, déjà décrites, illustrent un procédé connu pour réaliser des structures en nid d'abeilles ;
• les figures 2A et 2B, déjà décrites, illustrent un autre procédé connu pour réaliser des structures en nid d'abeilles ;
• les figures 3A et 3B, déjà décrites, illustrent encore un autre procédé connu pour réaliser des structures en nid d'abeilles ;
• les figures 4A à 4F illustrent différentes étapes successives d'un mode de mise en oeuvre du procédé selon l'invention pour fabriquer une texture plane en nid d'abeilles en matériau composite thermostructural ;
• les figures 5A à 5C illustrent la formation de peaux sur une structure en nid d'abeilles pour réaliser un panneau autoraidi ; et
• les figures 6A et 6B illustrent un autre mode de mise en oeuvre du procédé selon l'invention pour fabriquer une texture de révolution en nid d'abeilles.

Reference will be made to the appended drawings in which:

• FIGS. 1A, 1B and 1C, already described, illustrate a known method for producing honeycomb structures;
• FIGS. 2A and 2B, already described, illustrate another known method for producing honeycomb structures;
• FIGS. 3A and 3B, already described, illustrate yet another known method for producing honeycomb structures;
• FIGS. 4A to 4F illustrate different successive stages of an embodiment of the method according to the invention for manufacturing a flat honeycomb texture of thermostructural composite material;
• FIGS. 5A to 5C illustrate the formation of skins on a honeycomb structure to produce a self-supporting panel; and
• Figures 6A and 6B illustrate another embodiment of the method according to the invention for manufacturing a texture of revolution honeycomb.

A method according to the invention for producing a structure flat honeycomb made of thermostructural carbon / carbon type composite material is now described with reference to FIGS. 4A to 4F.

A first step in the process consists in producing a texture of three-dimensional carbon fiber reinforcement.

To this end, two-dimensional strata 40 made of carbon or a precursor carbon (for example polyacrylonitrile, or PAN, in the pre-oxidized state) are superimposed and needled (Figure 4A). Layers 40 are layers by example of fabric or complex formed of fabric and veil of fibers, the veil of fibers providing fibers which can be easily removed by the needles, during needling, to be implanted through the strata. Needling is carried out from preferably on fibers in the state of carbon precursor, needling performed directly on carbon fibers having a more destructive effect. For the realization needling, it can be carried out as the stacking of layers 40, as described in document FR-A-2 584 106, the thickness of the texture being determined as a function of that of the honeycomb structure to be produced.

Other strata linking techniques can be used, for example sewing, or even the implantation of threads as described in document FR-A-2,565,262.

When the three-dimensional texture 41 thus obtained is made of fibers in carbon precursor, a carbonization heat treatment must be carried out to transform the precursor into carbon. This treatment resulting in a slight withdrawal dimensional, it is preferably carried out before the realization, in the texture, of cut-outs or slots allowing the alveoli of the nest structure to form bees.

As shown in FIG. 4B, these cutouts 42 in the form of slots are made in staggered rows, their dimensions and locations defining the dimensions and shapes of the cells. The cuts 42 are made in planes parallel to each other and perpendicular to the planes of the strata 40. Preferably, the cut planes are parallel to one of the directions X and Y along which the warp threads and the weft threads are oriented. of the fabric of the strata 40, for example the direction X of the warp threads (the layers of fabric being superimposed with their parallel warp threads, as well, therefore, as their weft threads). Thus, the continuity of the warp (or weft) threads is preserved in the fabric layers after the cutouts have been formed. The cuts are all of the same length L and are regularly spaced by the same distance D in each plane. The planes are regularly spaced from each other by a distance d . The length L of the cutouts is greater than the length D of the interval between them and the staggered arrangement, in the example illustrated, is such that the middle of a cutout 42 in a cutout plane is at the level of the middle of the interval between two cuts 42 in the neighboring cutting planes.

The cuts 42 are made for example with a knife or a water jet.

After making the cuts, the texture 41 is stretched in direction Y perpendicular to the planes of the cuts (arrows F in Figure 4C).

Stretching results in a separation of the lips of the cutouts 42 (Figure 4C) and the formation of cells 46 whose walls are defined by these lips. Stretching is interrupted when the cells 46 have reached the desired shape (Figure 4D) and before the stresses exerted at the ends of the cuts cause a tear in the texture.

A fibrous honeycomb preform 47 is thus obtained in which the walls of the cells 46 are perpendicular to the planes X, Y of the strata 40.

It is possible to make the cuts 42 in planes inclined by compared to normal to strata 40. After stretching in direction Y, we then obtain alveoli whose walls are not perpendicular to the faces of the texture.

Tests carried out on textures such as that described above have shown that, during stretching, the walls of the cells remain perpendicular to the X, Y planes and that the surface deformations, induced in particular in the end zones of the cuts, remain of very low amplitude. He was also found that stretching does not cause tearing at the ends of the cuts. For comparison, tests carried out on textures identical to with the exception of needling (no connection between strata), have shown that stretching could cause damage to the texture at the ends of the cuts.

It should be noted that the deformation capacity of the texture 41 by pulling in direction Y presents a surprising character insofar as the fabrics are deemed to be non-deformable in their plan.

After stretching, the preform 47 is densified while being kept in the state stretched using a tool. The latter (Figure 4E) consists of a sole in graphite 50 and graphite pins 52 implanted in cells 46 along the opposite edges of the preform in Y direction. The pins 52 penetrate into holes formed in the bottom 50.

The assembly constituted by the tool 50, 52 and the preform 47 is introduced into an oven in which the preform 47 is densified by carbon by chemical vapor infiltration. In a manner well known in itself, a phase gas containing one or more hydrocarbons is introduced into the enclosure under conditions of temperature and pressure determined to promote a decomposition of the gas phase in contact with the fibers of the preform 47 and release carbon which gradually fills the porosity of the preform 47.

After densification, a honeycomb structure 48 is obtained in carbon / carbon composite (Figure 4F). Such a structure is likely to many applications. For example, it can constitute an oven sole of heat treatment and advantageously replace a metal sole obtained by molding or welding of elements. Such a honeycomb structure can also be used as a rigid tool for maintaining a preform at densify by chemical vapor infiltration, instead of tools traditional graphite.

Honeycomb structures in thermostructural composite material can also find applications to build autoraidis panels usable in aeronautical or space applications, for example as elements of space plane structures.

The honeycomb structure can, for certain applications, be with a skin on each side.

For this purpose, as shown in FIG. 5A, at least one layer fibrous 54, for example a layer of fabric, is stretched over the preform 47 held on the sole 50 by means of the pins 52. The layer of fabric 54 is needled on the edges of the cells 46 by means of a needling head whose displacements can be programmed, for example as described in the document FR-A-2 669 941.

After needling of the layer 54, the preform 47 and the fabric layer 54 a graphite sole 51 similar to sole 50 and provided with holes arranged to receive the upper ends of the pins 52 which protrude from the preform 47. The assembly is turned over and the sole 50 is removed to allow the placement of at least one other layer of fabric 55 stretched over the other face of the preform 47 and the needling of this other layer (FIG. 5B).

The assembly is then introduced into a chemical infiltration oven in vapor phase to simultaneously densify preform 47 and fabric layers 54, 55 needled on both sides, thereby obtaining a panel autoraidi 58 comprising a rigid honeycomb core 48 covered with two rigid skins 56, 57 closing the cells 46 (FIG. 5C).

When the self-supporting panel is not subjected to efforts important in shear, the fabric layers 54, 55 can be simply bonded to the faces of the preform 47 before being densified with it, the codensification completing the necessary connection between the skins and the soul of the sign.

In the foregoing description, it has been envisaged the realization of honeycomb structures in carbon / carbon composite material.

Of course, the invention is applicable to the production of structures in honeycomb in thermostructural composite materials other than composites carbon / carbon, in particular ceramic matrix composites whose reinforcement texture is in carbon or ceramic. The techniques used are those, known, of the realization of three-dimensional textures in fibers of carbon or ceramic, and densification by a ceramic matrix.

It will also be noted that the densification of the honeycomb preform, possibly provided with layers on its faces, can be carried out by liquid way, that is to say by impregnation using a precursor of the phase matrix liquid, then by transformation of the precursor. Several impregnation cycles, possibly supplemented by a chemical vapor infiltration cycle, may be required.

Finally, although we have described above the realization of structures in flat honeycomb, the invention is applicable to the production of structures curved or cylindrical. These can be obtained by shaping the honeycomb preform on an appropriate tool, before densification and stiffening.

It is also possible to make a three-dimensional reinforcement texture of revolution 60 by needling of layers 61 wound on a mandrel (Figure 6A), as described for example in document FR-A-2 584 107. Des staggered cuts 62 are formed in meridian planes, through any the thickness of the texture 60.

The cut texture is stretched on a mandrel 70 to form alveoli 66. Pawns 72, implanted in the mandrel 70, maintain the texture at the stretched state for its densification, for example by vapor phase infiltration (Figure 6B). After densification, a rigid cylindrical honeycomb structure is obtained.

Claims (10)

1. A method of manufacturing a honeycomb structure of thermostructural composite material comprising a fiber reinforcing fabric densified by a matrix, the fibers of the reinforcing fabric being of a material selected from carbon and ceramics, as is the matrix, the method being characterised in that it comprises the following steps:

   making a three-dimensional fiber fabric (41) by means of superposed two-dimensional plies (40) that are bonded together by means of fibers passing through the plies;

   making slit-shaped cuts in a staggered configuration (42) through the plies, and through the entire thickness of the fabric;

   stretching the cut fabric in a direction that is transverse with respect to the cuts (42) and that is parallel to the plies (40) so as to form cells (46) whose walls are constituted by the lips of the cuts (42); and

   while the cut fabric is held in the stretched state, densifying it using the matrix-constituting material to obtain a rigid honeycomb structure (48) of thermostructural material.
2. A method according to claim 1, characterised in that the two-dimensional pies (40) of the three-dimensional fabric (41) comprise layers of cloth.
3. A method according to claim 2, characterised in that the cuts (42) are made parallel to one of the following directions: the warp thread direction and the weft thread direction of the layers of cloth.
4. A method according to any one of claims 1 to 3, characterised in that the bonding between the plies (40) of the three-dimensional fabric is provided by needling.
5. A method according to any one of claims 1 to 4, characterised in that the cut and stretched fabric (42) is densified by chemical vapor infiltration while being held in the stretched state by means of tools (50, 52).
6. A method according to any one of claims 1 to 5, characterised in that the cut and stretched fabric (47) is provided with at least one fiber layer (54, 55) on each of its faces parallel to the plies of the fabric, and the assembly formed by the stretched fabric and the fiber layers is densified to obtain a self-rigid panel (58) comprising a rigid honeycomb core covered by a rigid skin on each face.
7. A method according to claim 6, characterised in that the fiber layer placed on each face of the fabric is bonded thereto by needling.
8. A honeycomb structure of thermostructural composite material comprising a fiber reinforcing fabric densified by a matrix, the fibers of the reinforcing fabric being of a material selected from carbon and ceramics, as is the matrix, the structure being characterised in that the reinforcing fabric is a three-dimensional fabric formed by two-dimensional plies (40) bonded together by fibers passing through the plies, the cells (46) of the honeycomb structure (48) being formed through the plies.
9. A honeycomb structure according to claim 8, characterised in that the two-dimensional plies (40) comprise layers of cloth which retain the continuity of the warp or weft threads.
10. A honeycomb structure according to any one of claims 8 and 9, characterised in that it comprises two rigid skins (56, 57) covering its faces parallel to the plies of the reinforcing fabric and closing the cells (46).

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